JP-A-2003-223997 proposes a high-pressure discharge lamp lighting device having a capability of deep dimming. FIG. 22 shows change rate of lamp power to voltage across lamp V1a. Circuit operates at the intersection point of the output characteristic of a lighting device indicated by a dotted line a, b in the figure and the lamp characteristic indicated by a heavy solid line c. If the circuit is thus operated, a constant-voltage characteristic can be taken in a comparatively high output dimming range (for example, 40% to 100%) until the voltage across lamp V1a turns to rise as shown in a in FIG. 22, and roughly constant power can be obtained for fluctuation of lamp impedance caused by the individual difference and the life of lamp; the circuit operates with a constant-current characteristic in a comparatively low output dimming range (for example, 40% or less) where the voltage across lamp V1a turns to rise as shown in b in FIG. 22, so that an operation point of an intersection point with the lamp characteristic can be taken, gong out does not occur, stable lighting can be performed, and the dimming range can be enlarged.
FIG. 23 shows target values of lamp current Ila in output characteristics a and b in FIG. 22. As described above, the target value of the lamp current Ila is decreased in response to an increase in the voltage across lamp V1a in the dimming area of the output characteristic a, thereby performing rough constant-power control, and the target value of the lamp current Ila is made constant in response to an increase in the voltage across lamp V1a in the dimming area of the output characteristic b, thereby performing constant-current control.
A discharge lamp lighting device of a characteristic as shown in JP-A-6-111987 is also known for the purpose of widening the dimming range. FIG. 24 shows the characteristic and shows lamp voltage V1a relative to lamp current Ila of the discharge lamp lighting device. Assuming that the current command value at the instant at which the lamp voltage V1a exceeds a threshold value Vth is X, if adjustment is made so as to output a current command value Y or Z lower than X, it crosses voltage-current curve Q of discharge lamp at a y point or a z point and the light state can be still maintained. Thus, when the lamp voltage V1a is lower than the threshold value Vth, constant-power dimming is performed, and constant-current dimming is performed in the range in which the lamp voltage V1a is equal to or greater than the threshold value Vth. This means that the dimming range is widened to a smaller luminance area.    Patent document 1: JP-A-2003-223997    Patent document 2: JP-A-6-111987